Digital radio frequency memory (DRFM) provides a means for receiving and subsequent digital regeneration of a radio frequency (RF) waveform. RF signals are received as analog signals. A DRFM converts the signal to a digital form using analog to digital converters (ADCs). The digital form of the received RF signal may then be stored in memory and processed digitally. Following processing, the digital form of the signal is sent through at least one digital to analog converter (DAC) that returns the signal to an analog form. The output is an analog signal representative of the originally received RF signal.
The waveform may be amplitude and/or frequency modulated and regenerated at a time corresponding with a desired (i.e. target) range offset. Based on these capabilities, a DRFM may be used as a test target generator or as a coherent repeater for example, in electronic warfare (EW) applications.
Conventional DRFMs are typically implemented via commercial off-the-shelf (COTS) converters. Those employing such devices, however, are faced with a tradeoff between bandwidth and spectral purity. As the instantaneous bandwidth of a DRFM increases, the spectral purity of the output decreases, limited by the sampling rate of the COTS ADCs and DACs.
FIG. 1 shows a conventional DRFM solution 100. An RF signal 105 is received at a high or intermediate frequency (IF). An ADC 110 samples the input waveform 105 at a sampling rate attainable by the ADC 110. The ADC 110 converts the analog samples into digital samples 115 for storage in memory 120. Once the digital samples 115 are stored in memory 120, digital processing in the form of a technique application 125 is performed. Technique application 125 may include performing any combination of (but not limited to) a frequency offset, a phase offset, and frequency, phase and/or amplitude modulation.
The technique application 125 receives a clock signal 135 from a DAC 140 which instructs the technique application 125 to send the processed digital samples 130 to the DAC 140. DAC 140 regenerates the original RF signal 145 in an analog form by converting the processed digital samples 130 to analog form in order to regenerate the original waveform 105. DFRM 100 is limited in bandwidth and spectral purity based on the ADC 110 and DAC 140 chosen. For example, the speed at which the ADC 110 may sample the received IF signal 105 may be at a rate that is lower than the frequency of the received signal 105. Thus, the ADC 110 does not support the bandwidth necessary to capture all the changes occurring in the incoming waveform 105. These limitations are carried through the process to the DAC 140, which also limits the degree of spectral purity of the output signal 145 being generated.
Accordingly, a DFRM that overcomes one or more of the above limitations, including the sampling speed of the ADC's and DAC's used to implement the DFRM, and allows for greater instantaneous bandwidth and higher spurious free dynamic range (SFDR), is desirable.